1. Field of the Invention
The present invention relates to delay circuits and more particularly is directed toward programmable digital delay circuits which have a variety of applications including, but not limited to, use in ultrasound imaging devices.
2. Description of the Prior Art
Ultrasound imaging devices are often used in examining a fetus, a particular internal organ or other object within a patient. The devices employ one or more electroacoustic transducers each of which has one or more small areas/faces otherwise known as elements which together form an aperature and from which ultrasound waves are propagated to form a beam of ultrasound energy. The beam is created by applying an electrical pulse across each element in order to cause a mechanical deformation of the latter resulting in the radiation of ultrasound waves from a preselected group of elements. For purposes of focusing and steering an ultrasound beam radiating from the preselected group of elements, the electrical pulses applied to the elements are staggered in time rather than applied to all of the elements at the same time. Such staggering requires that one or more elements within the preselected group be electrically energized (i.e. fired) at different times from one another through the use of delay circuits. These delay circuits are connected between a pulse generator and the elements whose firing times are to be delayed.
As the number of elements within the group increases, the capability for steering or focusing the beam increases. In addition, as the number of elements within the group increases, the depth at which an image has an acceptable resolution increases. Therefore, it is highly desirable to increase the number of elements within the group to be fired. Of course, as the number of elements increase, the number of delay circuits also will increase.
Delay lines used in delay circuits which are designed to delay the energization of transducers whose elements are arranged in the form of a linear array typically provide between 0-500 nanosecond delay periods and are normally rated with a tolerance of .+-.5 percent. Therefore, a delay line which is nominally rated at 500 nanoseconds may, in fact, delay an electrical pulse by as much as 525 nanoseconds or by as little as 475 nanoseconds. If two elements are to be fired after a 500 nanosecond delay but, in fact, due to the manufacturing tolerances of the delay lines fire at 525 and 475 nanoseconds, the corresponding ultrasound waves produced rather than being in phase with each other will be out of phase with each other by a 50 nanosecond deviation. As can be readily appreciated such manufacturing variations between the same nominally rated delay lines is quite undesirable and makes steering or focusing of the beam quite difficult if not impossible.
In order to more closely match the nominal rating of the delay line requires the manufacturing tolerances be greatly reduced to, for example, .+-.1 percent resulting in much more expensive delay lines. As discussed above, there is now a much greater emphasis on increasing the number of elements used for generating an ultrasouond beam. Accordingly, the cost of more expensive delay lines is much more unattractive, and may be cost prohibitive.